Monthly Archives: November 2008

astronomy

Seeing Other Worlds

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Astronomers Have the Images to Prove It

Astronomers are always on the hunt for planets in new and wonderful ways. The most productive method has been to locate planets around other stars by looking for their gravitational influence on a star’s light. That method results in interesting spectral plots (graphs), but doesn’t give you an image to look at.  As Bruce Macintosh of Lawrence Livermore National Laboratory recently put it, “When astronomers discover new planets around a star, all we see are wiggly lines on a graph of the star’s velocity or brightness.”

It’s important data, to be sure. But, we all know that a picture tells a thousand words, so of course, everybody would like to see actual images. Today, two teams of astronomers announced they’ve done just that: taken images of distant planets around other stars. One target was the star HR 8799, which lies in the constellation Pegasus and is visible through binoculars. These new images show that it has three planets, making this discovery the first time that astronomers have directly imaged a family of planets around another star.

But wait, as they say on the late-night TV ads, there’s more! Another group of astronomers made an image of a world orbiting the bright star Fomalhaut, familiar to stargazers as brightest star in the southern constellation Piscus Austrinus (Southern Fish). The exciting thing about THIS find is that Fomalhaut lies about 25 light-years away from us. In terms of our galactic neighborhood, that’s like finding a new house right around the corner!

The HR 8799 Planetary System

Lynnette Cooks space art conception of what the planets around HR8799 might look like.
Lynnette Cook's space art conception of what the planets around HR8799 might look like.

Astronomer Christian Marois and his colleagues used two of the most powerful ground-based infrared-sensitive telescopes on Earth to look at the HR 8799 system, which is a massive young star that lies about 130 light-years away.

They made an initial discovery of two of the planets in data obtained at Gemini on October 17, 2007. Subsequent observations made in October 2007 and in the summer of 2008 confirmed the discovery and found a third planet orbiting even closer to the star in images obtained using the Keck II telescope.

HR 8799 has about 1.5 times the mass of the Sun and is five times more luminous. However, it’s significantly younger, and infrared observations by satellites have shown evidence that it’s encircled by a massive disk of cold dust. It turns out to be one of the most massive dust disks in orbit around any star within 300 light-years of Earth. Just the kind of place where you would expect to see planets forming!

A direct image of the plaents around HR8799, made using adaptive optics and the Near-Infrared Imager at Gemini North Observatory.
A direct image of two of the three planets around HR8799, made using adaptive optics and the Near-Infrared Imager at Gemini North Observatory. "B" has about 7 Jupiter masses and orbits about 70 AU from the parent star. "C" is about 10 Jupiter masses and orbits at about 40 AU. "A" is not seen here, but it orbits about 25 AU from the star.

Now, since the star itself is fairly young, its planets are even younger. Astronomers estimate that they formed about sixty million years ago. They are still glowing from heat released as they contracted and that heat helped astronomers spot them in the infrared. Analysis of the brightness and colors of these worlds (at multiple wavelengths) shows that they are about seven and ten times the mass of Jupiter. The most distant of the planets (B) orbits inside the dust disk surrounding the planet, in a region that is similar to the Kuiper Belt in our own solar system. Astronomers consider this newly mapped system to be a scaled-up version of our own system, but orbiting a brighter star than we do.

HR 8799 observations are part of a survey of 80  young, dusty, and massive stars located in the solar neighborhood. The survey is being done using Gemini, the W.M. Keck, and VLT observatories, which are equipped with adaptive optics systems to do the searching. Adaptive optics systems basically remove the effects of Earth’s atmosphere by sending light through a series of deformable mirrors that compensate for the distortion that our atmosphere creates. To read more about how the teams got these remarkable images, surf on over to the Gemini website for details.

The scientists doing the work hope to find more planets around nearby stars, and perhaps ultimately they’ll be able to find another Earth-like planet out there.

A team led by Macintosh is constructing a much more advanced adaptive optics system designed from the beginning to block the light of bright stars and reveal even fainter planets. Known as the Gemini Planet Imager (http://gpi.berkeley.edu), this new system will be up to 100 times more sensitive than current instruments and able to image planets similar to our own Jupiter around nearby stars.

“I think there’s a very high probability that there are more planets in the system that we can’t detect yet,” Macintosh said. “One of the things that distinguishes this system from most of the extrasolar planets that are already known is that HR8799 has its giant planets in the outer parts – like our solar system does – and so has “room” for smaller terrestrial planets ­ far beyond our current ability to see ­ in the inner parts.”

Welcome to Fomalhaut B

What Fomalhaut b might look like.
What Fomalhaut b might look like.

So, what’s this about Fomalhaut’s planet?  For starters, it’s called Fomalhaut b, and it was found by astronomer Paul Kalas (of the University of California at Berkeley), who headed up a team that used Hubble Space Telescope to image the star and its dust disk.

Kalas suspected the existence of the planet when he began examining images of the star taken with HST’s Advanced Camera for Surveys in 2004 and 2006. They showed a sharply defined inner edge to the dust belt around Fomalhaut,. The sharp edge and off-center belt suggested to Kalas that a planet in an elliptical orbit around the star was shaping the inner edge of the belt, much like Saturn’s moons groom the edges of its rings.

“The gravity of Fomalhaut b is the key reason that the vast dust belt surrounding Fomalhaut is cleanly sculpted into a ring and offset from the star,” Kalas said. “We predicted this in 2005, and now we have the direct proof.”

Kalas’s two images showed the planet’s motion in its orbit. Fomalhaut b goes around its parent star once every 872 years at a distance of 119 astronomical units (one AU is the distance between Earth and the Sun).

The planet’s mass is a few times the mass of Jupiter and once the mass is confirmed, it will likely be the coolest and lowest-mass body imaged outside of our own solar system.  “It will be hard to argue that a Jupiter-mass object orbiting an A star like Fomalhaut is anything other than a planet,” said coauthor James R. Graham, professor of astronomy at UC Berkeley. “That doesn’t mean it’s exactly what we expected when we went hunting for planets in this system.”

This visible-light image, taken with the Advanced Camera for Surveys aboard HST, shows the newly discovered planet, Fomalhaut b, orbiting its parent star, Fomalhaut. The small white box pinpoints the planets location. Fomalhaut b has carved a path along the inner edge of a vast, dusty debris ring encircling Fomalhaut that is (34.6 billion kilometers (21.5 billion miles) across. Fomalhaut b lies 2.9 billion kilometers (1.8 billion miles) inside the rings inner edge and orbits 17.2 billion kilometers (10.7 billion miles) from its star. The inset is a composite image showing the planets position during Hubble observations taken in 2004 and 2006. Astronomers have calculated that Fomalhaut b completes an orbit around its parent star every 872 years.
This visible-light image, taken with the Advanced Camera for Surveys aboard HST, shows the newly discovered planet, Fomalhaut b, orbiting its parent star, Fomalhaut. The small white box pinpoints the planet's location. Fomalhaut b has carved a path along the inner edge of a vast, dusty debris ring encircling Fomalhaut that is (34.6 billion kilometers (21.5 billion miles) across. Fomalhaut b lies 2.9 billion kilometers (1.8 billion miles) inside the ring's inner edge and orbits 17.2 billion kilometers (10.7 billion miles) from its star. The inset is a composite image showing the planet's position during Hubble observations taken in 2004 and 2006. Astronomers have calculated that Fomalhaut b completes an orbit around its parent star every 872 years.

Astronomers are studying interactions of the planet with the dust belt surrounding the star and helps them estimate the mass of the planet.  “Every planet has a chaotic zone, which is basically a swath of space that encloses the planet’s orbit and from which the planet ejects all particles,” said Eugene Chiang, a UC Berkeley associate professor of astronomy and of earth and planetary science. “This zone increases with the mass of the planet, so, given the size of the chaotic zone around Fomalhaut b, we can estimate that its likely mass is in the vicinity of one Jupiter mass.”

There are many more fascinating details about this find, and I encourage you to surf over the HST site and the European ESA/HST site for more information. There are more images and instrument descriptions, and some really cool videos, too! If you have subscriptions to the journal Science and Science Express (part of Science), these results appear there starting today.

These are exciting days for planetary discovery teams. The advent of highly sensitive instruments on ground-based telescopes, coupled with the kinds of clear images that Hubble Space Telescope can provide are bringing what planet-search folks have always dreamed of: high-resolution images of worlds around distant neighboring stars. It goes a long way towards showing that our solar system is not alone in the cosmos.

exoplanets, extrasolar planets, planet-forming scenarios

Kick-starting Planet Formation

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Shock and Ahhhh in Circumstellar Disks

There’s some big news coming up tomorrow about exoplanets (planets around other stars) which I think people will find quite tasty. But, before we get to that story (which will be released tomorrow afternoon), let’s talk about the ingredients of planets. It turns out making them requires quite a complex mix of elements and processes. When it works right, it’s quite satisfying because you get new worlds to explore.

Crystals like this were likely formed in shock waves in dust disks around the Sun and other stars.
Crystals like this were likely formed in shock waves in dust disks around the Sun and other stars.

First you have to start out with some starbirth. This star has to be forming with a dusty disk around it. Planets are born in these swirling pancake-like disks. They start out as mere grains of dust swimming around in the disk before lumping together to form full-fledged planets. During the early stages of planet development, the dust grains crystallize and adhere together, while the disk itself starts to settle and flatten out. This occurs in the first millions of years of a star’s life.

Okay, so we know that these circumstellar disks provide the raw materials for planets, moons, rings, asteroids, and comets. What are they? Typically you have gases (hydrogen, oxygen, etc.) You also have ices made of water, methane, and ammonia (to name the most abundant). And, you have elements such as iron, carbon, and silicon, and as has been detected recently by Spitzer Space Telescope, crystals of materials such as silica, which — if exposed to a significant amount of heat to melt them — can form other crystals called cristobalite and tridymite (which are often found around volcanoes here on Earth).

So, what could be causing the heating that would melt and crystalize silica?  Astronomer WIlliam Forrest of the University of Rochester in New York, used Spitzer to studied material around other star systems and found the chemical signatures of these crystals, which need a quick, high-temperature environment to form in. “Cristobalite and tridymite are essentially high-temperature forms of quartz,” said Sargent. “If you heat quartz crystals, you’ll get these compounds.”

In fact, the crystals require temperatures as high as 1,220 Kelvin (about 1,740 degrees Fahrenheit) to form. But young planet-forming disks are only about 100 to 1,000 Kelvin (about -280 degrees Fahrenheit to 1,340 Fahrenheit) — too cold to make the crystals. They need a hot environment followed by rapid cooling.

So, how do you create the ovens of crystal formation in such chilly places?  Shock waves. These fast-moving (supersonic, in fact) pressure waves are likely to occur when clouds of gas in the disks collide. Those clouds are swirling around at high speed and when they smack into each other, it could raise the temperature enough to create those crystals.  You can also get such shock waves when giant planets like Jupiter are formed.

A dust disk around the star AU Mic (courtesy M. Liu, IfA-Hawaii/Keck Observatory)
Astronomers have found many stars with dust disks around them. This is one circles the star AU Mic. It shows lumpy structure that could be a clue to the existence of planets embedded inside. (courtesy M. Liu, IfA-Hawaii/Keck Observatory)

The planet-building process is a long one, taking millions of years at a minimum (if our own solar system is any example). It starts with the cloud of gas and dust swirling around in space, with a protostar in the center. As things heat up, things fuse and stick together to form these crystals. In the outer regions, where there’s not so much heat, ice cFrom smaller crystals you get bigger crystals and clumps of rock that fuse together over long periods of time to make even bigger clumps. Eventually, if you slam enough of them together, you get asteroids that clump together to make small worlds, which clump together to make larger worlds… and… you get the idea.

And, we see the evidence for similar processes here in our own solar system. Spherical pebbles called “chondrules” have been found in ancient meteorites that we know formed in the early days of the solar system. They bear evidence of shock processes similar to the ones that astronomers have found evidence for at distant stars.It’s pretty likely that, like those distant stars and their planets, our own solar system experienced the heating events that kick-started the recipe for planets more than 4.5 billion years ago. This is the kind of research discovery that helps astronomers put together larger, much more satisfying (and “ahhh”-inducing) scenarios for planetary formation.

And the hunt for planets continues. For the first time in history, we have enough observational firepower, both in space and on the ground, to make some big discoveries of planets AND hone in on the details of planet-forming processes. So, check back tomorrow (Thursday) afternoon and I’ll have some more big news on the planet-discovery front!